Safety relief valve having a low blow-down value and spring therefore

ABSTRACT

A safety relief valve is provided that comprises a body comprising a chamber therein and an inlet and an outlet, the inlet comprising an inlet valve seat; a disk member closable on the inlet valve seat; and a mechanism in the body biasing the disk member to rest on the inlet valve seat with a set force such that when the pressure in the inlet exceeds a set pressure resulting from the set force, the disk member is lifted from the inlet valve seat, the mechanism comprising a combination of a coil spring and one or more disk springs stacked in series with the coil spring. A method of preparing a coil spring for use in a pressure relief valve comprises measuring the spring rate of the coil spring and; modifying the spring so as to modify its spring rate to be within ±2% of a desired spring rate.

REFERENCE TO EARLIER FILED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/364,290, filed Feb. 10, 2003, which is acontinuation-in-part of U.S. patent application Ser. No. 09/885,293,filed Jun. 19, 2001, now U.S. Pat. No. 6,516,828, all of which arehereby incorporated by reference in their entirely.

BACKGROUND

The present invention relates to safety relief valves for use on apressurized system such as pressure vessel or a flow line, especiallysnap-type safety relief valves having a consistent low blow-down value,springs for safety relief valves and methods of manufacturing safetyrelief valves.

Spring operated relief valves are used to protect pressurized systemsform pressures that exceed their maximum allowable working pressure orany pressures that the user specifies. Most spring operated reliefvalves use an externally adjustable coil springs that, when compressed,applies the force to keep the valve closed. The spring, which isexternally adjustable by the means of a threaded adjustment screw, canbe set to allow the valves to operate at a wide range of pressures. Thepressure that an individual valve is set to open at is called the setpressure. When this set pressure is reached the valves opens andrelieves the excess pressure. The valve then closes when the systempressure has dropped to a reduced level.

Snap-type safety relief valves have the advantage of responding veryquickly to pressure changes in pressurized systems to which they areattached. Snap-type safety relief valves move to a fully open positionalmost immediately after the pressure within the pressure vessel risesabove the user-determined set pressure. This allows excess gas pressureto escape quickly. Then, when sufficient pressure has escaped, snap-typesafety relief valves quickly and crisply move back to a closed position.For an example of a snap-type safety relief valve, see U.S. Pat. No.3,664,362, which is herein incorporated by reference in its entirety.

A “blow-down value” is the percentage difference between theuser-determined set pressure and the pressure in the pressure vessel orflow line when the snap-type safety relief valve snaps closed. Forexample, if a user sets the set pressure at 100 psi, and valve staysopen as gas escapes out of the snap-type safety relief valve until thepressure in the pressure vessel or flow line is 70 psi, then thissnap-type safety relief valve has a blow-down value of 30%. For anexample of a snap-type safety relief valve having a standard blow-downvalue, see U.S. Pat. No. 4,799,506, which is herein incorporated byreference in its entirety. Low blow-down valves have a blow-down valueof about 15% or less, preferably about 10% or less. A particularlypreferred valve will have a blow-down of 5% to 10% of set pressure. Ifthe set pressure were, for example 100 psi, the reseat pressure wouldfall in the range of 90 psi to 95 psi for such a preferred valve. Lowblow-down valves are desirable because they can minimize the amount ofgas that is lost from the pressurized system into the atmosphere duringventing, thereby addressing environmental concerns.

Existing low blown-down snap-type safety relief valves do, however, havesome problems. One problem is that the blow-down values of the valvesare affected by built-up downstream back pressures. The term “built-updownstream back pressures” is well understood in the art and documentedin the American Petroleum Institute Recommended Practice 520. The lengthof outlet piping and the number of elbows that are attached to theoutlet of the snap-type safety relief valves contributes to built-updownstream back pressures. Generally, the longer the outlet piping andthe greater the number of elbows in the outlet piping, the more built-updownstream back pressure is are created.

Built-up downstream back pressures affect the blow-down value of typicalsnap-type safety relief valves. For example, a manufacturer may sell asnap-type safety relief valve with a blow-down value of 10% that isrecommended to be used with 10 feet of outlet piping. At an installationsite, the installer may disregard the manufacturer's recommendations anduse 20 feet of outlet piping. In such a case, when the valve is in use,it will experience greater built-up downstream back pressures than themanufacturer designed for. The additional built-up downstream backpressures counteract forces that keep the valve open, and may cause thesnap-type safety relief valve to close prematurely. If this occurs whilethe pressurized system still needs to vent, the snap-type safety reliefvalve would open again. The valve may then open and close in rapidsuccession, which is a phenomenon known as chatter. Chatter can shortenthe life of a snap-type safety relief valve.

Another problem with existing low blow-down snap-type safety reliefvalves is that they tend to have a sliding-fit piston/sleeve design.See, for example, the snap-type safety relief valve described in U.S.Pat. No. 3,411,530, which is herein incorporated by reference in itsentirety. In these designs, when the piston is raised so that gas mayescape, a portion of the sleeve may obstruct the flow path as fluidflows through holes in the sleeve. As fluid escapes, foreign particlestend to accumulate between the sliding surfaces, causing additionalfriction between the sliding-fit parts. This build-up of foreignparticles can cause freeze-up of the piston. This can affect the amountof pressure necessary to open the valve, and it can affect the blow downvalue of the valve, making the valve's performance less predictable.Also, contaminated gas can cause the valve to malfunction.

In addition to the above-mentioned problems, low blow-down snap-typesafety relief valves can vary in quality in a number of ways. Allsnap-type safety relief valves each have a flow coefficient, whichrepresents how unobstructed gas flows through the valve when it is fullyopen. Higher flow coefficients are considered to be better. Also,different snap-type safety relief valves vary in their ability tomaintain their blow-down value, their performance reliability, theirdurability, their cost to manufacture, and their ease of use.

It is well known in relief valve art that for any given valve design,such as that disclosed in parent application Ser. No. 09/885,293, thatas set pressures increase, the slope of the “Force-lift Curve” becomessteeper, i.e., the ratio of force to lift becomes larger. A detaileddiscussion of the “Force-lift Curve” is provided in U.S. Pat. No.4,799,506, particularly in reference to FIG. 6 of that patent.

To obtain consistently low blow-downs over a broad range of setpressures for a given valve model, say 80 psi to 1500 psi, a series ofsprings, typically numbering from 15-20, is required.

A single spring in a relief valve will produce a range of operation forone size of valve, which is dependent on the rate of the spring. As therate of the coil spring increases, the blow-down will also decrease. Fora standard relief valve this range is quite large, achieving 20%blow-down at a set pressure and then 40% blow-down at some higher setpressure. In this type of valve line, the next higher pressure valvewould be made with a higher rate spring that would achieve a blow-downof 20% at the same pressure that the previous valve produced a 40%blow-down. This is then repeated throughout the entire valve line sothat when the individual valves are put together, they will cover arange of, for example, 15-2500 (psig). Because of standard spring ratetolerance these valves are designed with overlapping pressure ranges,which results in a slightly reduced operating range. This ensures that,even though springs rate tolerances drift, each valve will functionproperly over its intended operating range. Standard blow-down reliefvalves have very large spring ranges so that this overlap onlynecessitates the availability of a few sizes of springs in the inventoryused to make the valves. Low blow-down valves, on the other hand, haveoperating ranges so short that any small over lap will greatly increasethe number of springs needed for the valve line. In fact the overlapneeded for a ±7% tolerance spring is so large that a 5%-10% blow-downvalve line is not practical. A spring with a rate tolerance of ±5% wouldbe about the maximum rate drift for this type of valve line. A ratetolerance of no greater than ±2%, ideally no greater than ±1%, wouldallow for a practical amount of springs in this line. This is anunrealistic tolerance due to spring manufacturing limitations and cost.

For example the following springs might be typical for a 0.788 in.diameter orifice safety relief valve, where increased wire sizes areused to keep the maximum allowable stress below an acceptable designlimit. Lift values in ASME code-designed valves are constant over theset pressure range of any given orifice size. Set Point Ranqe SpringRate Wire Size 77.5-97.5 psi 80 lbs/in. .125 in. 97.5-120 psi 104lbs/in. .125 in. 120-145 psi 128 lbs/in. .136 in. 145-173 psi 154lbs/in. .136 in. 173-203 psi 182 lbs/in. .148 in. 203-240 psi 214lbs/in. .168 in. 240-234 psi 248 lbs/in. .177 in. 337-403 psi 289lbs/in. .177 in. 403-485 psi 335 lbs/in. .177 in. 485-585 psi 388lbs/in. .187 in. 585-720 psi 450 lbs/in. .203 in. 720-840 psi 610lbs/in. .203 in. 890-1120 psi 714 lbs/in. .225 in. 1120-1420 psi 838lbs/in. .225 in. 1420-1840 psi 990 lbs/in. .250 in. 1840-2420 psi 1175lbs/in. .250 in.

In order to insure that spring ranges do not over lap, and meet aspecification of a 5% to 10% blow-down for each spring, very precisecontrol of the rate of any given spring is required. Again refer to FIG.6 of U.S. Pat. No. 4,799,506.

Helical or coil springs are one of the most common types of springs andare produced by the millions each year. Coil compression springs produceresistance to a compressive force that is applied through its centralaxis. These springs, when coiled with a constant diameter, produce aresistive force that is directly proportional to the spring'sdisplacement. This property makes coil springs very predictable and easyto work with. The term rate, which is represented by R, describes thespring's ability to resist a given force. This can be shownmathematically by the equation: R=F/D where F is force acting on thespring and D is the displacement due to that force. For springs thathave a constant diameter and wire size, the rate is constant or linear,which mean that at any point of displacement the spring's rate stays thesame. Constant rate coil springs are designed to a desired rate, whichis dependent on four factors: wire size and modulus, the spring's meandiameter, and number of coils. Deviations and inconsistencies in thesefour factors make spring production not an exact science. Springtolerance will vary form one spring manufacturer to another, but ingeneral as design tolerances decrease so does the cost of producing thesprings.

Most coil springs are wound on a highly efficiently machine that canproduce standard springs very cheaply. For most spring manufactures,+10% deviations in spring rates is about average, and a rate around ±7%is considered about as tight as possible. Other types of coil springmanufacturing are available, such as springs that are machined for ablank. These machined springs can be produced to tighter tolerances thanconventional springs, but are extremely expensive compared to wound coilsprings. Because of this high rate tolerance, most spring applicationshave allowances for rate deviations, but in the safety relief valveindustry, these deviations can be problematic for low blow-down valves.Spring operated relief valves are mass-produced with standard springsand, because of design allowances, work fine. Low blow-down reliefvalves with short spring ranges, on the other hand, need as tight of aspring rate tolerance as possible.

One approach to obtaining a rate tolerance of ±2% would be to test allsprings from a good commercial spring vender, and discard those thathave rates outside of the ±2% tolerance. This, of course, is verywasteful. In order to realize a 5% to 10% blow-down, as many as 40different springs would have to be designed and purchased to cover theset pressure range for a given orifice size. The rate for each springwould then be recorded and sorted. To fill an order for any set pressureover the range of 80 psi to 1800 psi, a specific spring would then beselected with the rate required to produce a relief valve with ablow-down in the 5% to 10% range. While the procedure could beeffective, it would be inefficient from a spring inventory andmanufacturing perspective.

Thus there is a need for a way to modify springs for use in lowblow-down valves so that they will have a rate within a ±2% tolerance.Also, a low blow-down valve design is needed to accommodate suchmodified springs.

SUMMARY OF THE INVENTION

A snap-type safety relief valve has been discovered that addresses theproblems left unsolved by prior valves. Namely, it has been discoveredthat a snap-type safety relief valve having sonic flow into the body ofthe valve and out of a secondary orifice on the valve prevents built-updownstream back pressures from affecting the low blow-down value of thevalve. This means that the addition of longer outlet piping on thevalve, within limits, will not change the low blow-down value of thevalve. Thus, the low blow-down value of the valve is consistent. If anexcessive length of piping is added, however, then the flow out of thesecondary orifice will no longer be sonic, and the blow-down value willbe subject to built-up downstream back pressures. Nevertheless, so longas the flow into the body of the valve and out of the secondary orificeof the valve is sonic, built-up downstream back pressures are preventedfrom counteracting the forces keeping the valve open. The snap-typesafety relief valve of the present invention preferably has a high flowco-efficient, and is easy and inexpensive to use and manufacture.

Also, it has been discovered that if spring rate tolerances are reducedfrom +7% to +2%, the number of different springs for a given valvedesign can be reduced from approximately 40 to 20. Furthermore, standardspring ranges can be established and documented to vastly improve themanufacturing process and also assist field servicing. Thus an improvedsafety relief valve using commercially available springs modified byvarious methods is also the subject of the present application.

In one aspect of the invention, a snap-type safety relief valve designedfor use under specified operating conditions on a pressure vessel or aflow line having a pressurized gas therein is provided. The safetyrelief valve includes a body attachable to the pressure vessel or flowline, the body comprising a chamber therein and an inlet and an outlet,the inlet comprising an inlet valve seat, the outlet being adapted toattach to outlet piping. The safety relief valve also includes a diskmember closable on the inlet valve seat. Further, the safety reliefvalve includes a mechanism in the body biasing the disk member to reston the inlet valve seat with a set force such that when the pressure inthe pressure vessel exceeds a set pressure resulting from the set force,the disk member is lifted from the inlet valve seat. The safety reliefvalve also includes a secondary orifice between the body chamber and theoutlet, the secondary orifice being sized so that gas flows from theinlet valve seat into the chamber in a sonic flow and so that gas flowsfrom the chamber through the secondary orifice in a sonic flow when thevalve opens due to a pressure in the pressure vessel or flow lineexceeding the set pressure during testing under the specified operatingconditions.

In a second aspect, the invention is a safety relief valve comprising abody comprising a chamber therein and an inlet and an outlet, the inletcomprising an inlet valve seat; a disk member closable on the inletvalve seat; and a mechanism in the body biasing the disk member to reston the inlet valve seat with a set force such that when the pressure inthe inlet exceeds a set pressure resulting from the set force, the diskmember is lifted from the inlet valve seat, the mechanism comprising acombination of a coil spring and one or more disk springs stacked inseries with the coil spring.

In a third aspect, the invention is a method of manufacturing a group ofsafety relief valves each with a blow-down value of less than about 10%,the group covering a range of set pressures, the method comprisingmanufacturing multiple bodies, each having a chamber within the body, aninlet and an outlet, the inlet comprising an inlet valve seat; fitting adisk member within each of the bodies so as to be closeable on the inletvalve seat within the body; obtaining a plurality of coil springs to beused in the group of safety relief valves, each spring having anoriginal spring rate, the original spring rates varying within theplurality of springs; measuring the original rate of each spring; andassembling the valve by placing a given spring within a given body so asto act with an adjustable force on the disk member, the given springhaving its original spring rate modified prior to the final assembly ofthe valve.

In a fourth aspect, the invention is a method of preparing a coil springfor use in a pressure relief valve comprising measuring the spring rateof the coil spring and; modifying the spring so as to modify its springrate to be within ±2% of a desired spring rate.

In a fifth aspect, the invention is an improved low blow down safetyrelief valve having an inlet, a disk member closable on the inlet, amechanism biasing the disk member on the inlet, a body, and an outlet,the improvement comprising the mechanism comprising a spring having amodified spring rate.

In another aspect of the invention, a combination of outlet piping and asnap-type safety relief valve is provided. The combination includes asafety relief valve body attachable to the pressure vessel or flow line,the body comprising a valve body chamber therein and an inlet and anoutlet, the inlet comprising an inlet chamber upstream of an inlet valveseat, the outlet being attached to the outlet piping, a portion of theoutlet piping creating an outlet chamber proximal to the outlet. Thecombination also includes a disk member closable on the inlet valve seatand an adjustment screw acting on a spring in the body biasing the diskmember to rest on the inlet valve seat with a set force such that whenthe pressure in the pressure vessel exceeds the set pressure resultingfrom the set force, the disk member is lifted from the inlet valve seat.The combination further includes a secondary orifice between the valvebody chamber and the outlet, the secondary orifice being sized so thatpressure in the valve body chamber is less than about 50% of thepressure in the inlet chamber and so that pressure in the outlet chamberis less than about 50% of the pressure in the valve body chamber, whenthe valve opens due to a pressure in the pressure vessel or flow lineexceeding the set pressure during testing under the specified operatingconditions.

In still another aspect of the invention, an improved low blow-downsnap-type safety relief valve is provided. The valve has an inlet, adisk member closable on the inlet, a mechanism biasing the disk memberon the inlet, a body, and an outlet. The valve prevents built-updownstream back pressures from changing a blow down value. Theimprovement on the valve comprises a secondary orifice in the body ofthe safety relief valve sized to permit gas to escape from the bodythrough the secondary orifice in a sonic flow when the valve is openedduring testing under operating conditions specified for the use of thevalve.

In yet another aspect of the invention, a method of designing a lowblow-down snap-type safety relief valve is provided. The methodcomprises (a) choosing a set of operating conditions under which thevalve will ordinarily be run, including a set pressure; (b) providing atrial snap-type safety relief valve having: (i) a body attachable to thepressure vessel, the body comprising a chamber and an inlet and anoutlet, the inlet comprising an inlet valve seat, the outlet beingadapted to attach to the outlet piping; (ii) a disk member closable onthe inlet valve seat; (iii) a mechanism in the body biasing the diskmember to rest on the inlet valve seat with a set force such that whenthe pressure in the pressure vessel exceeds the set pressure resultingfrom the set force, the disk member is lifted from the inlet valve seat;and (iv) a secondary orifice between the chamber and the outlet, thesecondary orifice having a first diameter; and (v) the snap-type safetyrelief valve having a first pressure gauge attached thereto, thesnap-type safety relief valve being mounted on a pressure vessel, thepressure vessel having a second pressure gauge attached thereto; (c)causing the snap-type safety relief valve to snap open by increasing thepressure in the pressure vessel to exceed the set pressure; and (d)comparing a reading from the first pressure gauge to a reading from thesecond pressure gauge while the valve is open, (i) if the reading of thepressure of the first pressure gauge is from about 35% to about 50% ofthe pressure of the second pressure gauge, then the design process iscomplete; else (ii) if the reading of the pressure of the first pressuregauge is not from about 35% to about 50%, adjusting the diameter of thesecondary orifice and repeating steps (c)-(d) until the reading of thefirst pressure gauge is from about 35% to about 50% of the reading ofthe second pressure gauge.

In still another aspect of the invention, a method of relieving pressurefrom a pressurized system is provided. In this method, a snap-typesafety relief valve is connected in fluid communication with thepressurized system and the valve has (i) a body surrounding a chamber,(ii) an inlet with an inlet valve seat, (iii) a disk member closeable onthe inlet valve seat, (iv) a mechanism biasing the disk member closed onthe inlet valve seat, (v) an outlet attachable to outlet piping, and(vi) a secondary orifice between the chamber and the outlet. The methodcomprises exceeding a set pressure established for the valve, therebylifting the disk member from the inlet valve seat. The method alsocomprises flowing gas from pressurized system through the inlet into thechamber in a sonic flow. The method also comprises flowing gas from thechamber through the secondary orifice into the outlet in a sonic flow.

The present invention provides the foregoing and other features, and theadvantages of the invention will become further apparent from thefollowing detailed description of the presently preferred embodiments,read in conjunction with the accompanying drawings. The detaileddescription and drawings are merely illustrative of the invention and donot limit the scope of the invention, which is defined by the appendedclaims and equivalents thereof.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first preferred embodiment of alow blow-down snap-type safety relief valve in a closed position.

FIG. 2 is cross-sectional view of a second preferred embodiment of a lowblow-down snap-type safety relief valve in a closed-position.

FIG. 3 cross-sectional view of a third preferred embodiment of anembodiment of a low blow-down snap-type in a closed position.

FIG. 4 is cross-sectional view of a nozzle that can be used with any ofthe low blow-down snap-type safety relief valves of FIG. 1-3.

FIG. 5 is a top view of the nozzle of FIG. 4.

FIG. 6 is a schematic view of a preferred embodiment of a low blow-downsnap-type safety relief valve mounted on a pressure vessel.

FIG. 7 is a cross-sectional view of a fourth preferred embodiment of alow blow-down snap-type safety relief valve in a closed position,depicting the use of Belleville washers to produce a reduced toleranceon the rate of the set spring.

FIG. 8 is a cross-sectional view of a fifth preferred embodiment of alow blow-down snap-type safety relief valve in a closed position,depicting an internal spiral cylinder that is used to short out coilsand increase the rate of a coiled spring.

FIG. 9 is a cross-sectional view of a sixth preferred embodiment of alow blow-down snap-type safety relief valve in a closed position,depicting a basic coil spring modified by the addition of a series ofBelleville washers to reduce the overall spring rate tolerance from +7%to +2%.

FIG. 10 is a cross-sectional view of a seventh preferred embodiment of alow blow-down snap-type safety relief valve in a closed position,depicting a basic coil spring modified by the reduction of the springoutside diameter to reduce the rate tolerance from +7% to +2%.

FIG. 11 is a cross-sectional view of an eighth preferred embodiment of alow blow-down snap-type safety relief valve in a closed position,depicting a basic coil spring modified by enlargement of the springinside diameter to reduce the rate tolerance from +7% to +2%.

FIG. 12 is a cross-sectional view of a ninth preferred embodiment of alow blow-down snap-type safety relief valve in a closed position,depicting a basic coil spring wherein the inside diameter is enlarged bymechanical means to reduce the rate tolerance from ±7% to ±2%.

DETAILED DESCRIPTION OF THE DRAWINGS AND OF THE PREFERRED EMBODIMENTS OFTHE INVENTION

“Pressure” as used herein generally refers to absolute pressure ratherthan gauge pressure, unless otherwise specified. Pressures measuredrelative to zero pressure (a vacuum) are absolute pressures. Absolutepressure is used in the perfect gas law. Pressures measured relative toatmospheric pressure are called gauge pressures. Absolute pressure (psiaor pounds per square inch absolute) equals the measured gauge pressure(psig) plus atmospheric pressure.

Referring to FIG. 1, a low blow-down snap-type safety relief valve 10 isshown in a closed position. The low blow-down snap-type safety reliefvalve 10 is removably affixed to a pressure vessel 15. Any method knownin the art can be used to removably affix the snap-type safety reliefvalve 10 to the pressure vessel 15. Preferably, a threaded engagement isused. Nuts, bolts, screws, adhesives, and friction fitting may also beused.

The snap-type safety relief valve 10 has a body 18 defining a chamber 20therein. Preferably, the body 18 is made of steel. Preferably, the steelthat is used in the body 18 is one or more of commercially availablemild steel, stainless steel, carbon steel, and combinations thereof. Thematerial for a particular valve 10 may be selected based upon what thevalve 10 will be used for, in particular, which gas will be flowingthrough valve 10.

In the closed position, disk member 28 removably rests upon the inletvalve seat 26 in the inlet nozzle 25. Preferably, the disk member 28 ismade of steel. Preferably, the steel that is used in the disk member 28is one or more of commercially available mild steel, stainless steel,carbon steel, and combinations thereof. Generally, the types of diskmembers 28 that are preferred require few sliding-fit parts, becausesliding-fit parts and their assemblies are subject to high wear andtear. For example, the type of disk member 28 that is described in U.S.Pat. No. 4,799,506, which is herein incorporated by reference in itsentirety, is a preferred disk member 28.

When the disk member 28 rests on inlet valve seat 26, gas cannot escapefrom the pressure vessel 15 through the inlet 16 and out of the inletvalve seat 26 into the chamber 20. The disk member 28 rests on inletvalve seat 26 in the closed position because a user has adjusted amechanism to bias the disk member 28 to stay closed on inlet valve seat26. Preferably, the mechanism is an adjustment screw 35, which acts,directly or indirectly, against a spring 38 to hold disk member 28 oninlet valve seat 26. For example, in FIG. 1, the adjustment screw 35causes member 37 to directly act on spring 38.

The amount of force that the adjustment screw 35, member 37, and thespring 38 apply against the disk member 28 produces the set pressure.Although an adjustment screw 35 and a spring 38 are preferred, the setpressure can be adjusted using any mechanism known in the art.

Adjustment screw 35 is particularly preferred because it has a bore 39therein. When the disk member 28 is removed from the inlet valve seat26, the bore 39 provides a vertical path of movement for a stem on thedisk member 28. Using this type of adjustment screw 35, the disk member28 is guided up and down without any parts obstructing gas flow out ofthe snap-type safety relief valve 10, and without any undesirablelateral motion. This type of adjustment screw 35 is fully described inU.S. Pat. No. 4,799,506.

When the pressure inside the pressure vessel 15 exceeds the setpressure, snap-type safety relief valve 10 opens. Specifically, the gascomes through the inlet 16 through the inlet nozzle 25 and applies anupward force to the disk member 28. When the upward force exceeds thedown forces acting on disk member 28, including the set pressure, thedisk member 28 is lifted from the inlet valve seat 26 and guided upward.Note that the upward/downward language is used to describe theorientation of preferred embodiment of the valve 10 shown in FIG. 1. Thesnap-type safety relief valve 10 could be designed so that the diskmember 28 moved horizontally rather than vertically, or in anotherdirection.

When the disk member 28 is lifted from the inlet valve seat 26, gasescapes from the inlet nozzle 25 into the chamber 20. The gas escapesinto the chamber 20 a sonic flow, which is the fastest that the gas canescape into valves made with a standard nozzle. The pressure in thechamber 20 is less than about 50% of the pressure in the inlet chamber16. Even if the pressure in chamber 20 is much less than about 50% ofthe pressure in the pressure vessel or flow line (say, 30%), thepressure drop from the inlet chamber 16 to the throat of the inletnozzle 25 will stay at about 50%.

The term “about 50% is used because the critical pressure ratio for mostgases used is fairly close to 50%. For example, the critical pressureratios for air, nitrogen, and oxygen are all 52.8%, the criticalpressure ratio for natural gas is 55.1%, and the critical pressure ratiofor propane is 57.6%. Thus, one of skill in the art should understandthat the term “about 50% is meant to reflect the critical pressure ratiofor the gas used in a particular installation of a snap-type safetyrelief valve 10.

Next the gas must flow from the chamber 20 out of the secondary orifice30, which lies in the orifice plate 40, out of the outlet and into theoutlet piping 50. Flow out of the secondary orifice will be sonic whenthe valve is fully open under specified operating conditions, which willprevent built-up downstream back pressures from causing the valve toclose prematurely. This means that the portion of the outlet piping 50that is proximal to the secondary orifice 30, an outlet chamber 47, musthave a pressure therein that is less than about 50% of the pressure inthe chamber 20.

The orifice plate 40 is preferably made of steel. In steel embodiments,the orifice plate 40 is preferably stainless steel. The orifice plate 40can be attached to the body 18 of the snap-type safety relief valve 10using any mechanism known in the art, including friction fitting, nuts,bolts, screws, and the application of adhesive substances.

In FIG. 1, without the orifice plate 40, the gas would flow out of theoutlet in a subsonic manner, making the snap-type safety relief valve 10susceptible to built-up downstream back pressures caused by the lengthand shape of the outlet piping 50.

By adding the orifice plate 40 to the body 18, the area of the secondaryorifice 30 through which the gas can flow has been reduced. The area ofthe secondary orifice 30 should be chosen to create conditions that willallow sonic flow out of the secondary orifice 30.

The desired area of secondary orifice 30 depends on the specifiedoperating conditions for a particular installation of the snap-typesafety relief valve 10. Such specified operating conditions include butare not limited to the media (the gas) being relieved through the valve10, the temperature at which the valve 10 is run, the maximum pressureof the pressure vessel or flow line to which the valve 10 will beattached (which will be related to the set pressure), the size of theinlet and the outlet of the valve 10, the material comprising the trimof the inlet nozzle 25, the maximum flow capacity of the valve 10, thelength and number of elbows of the outlet piping 50 attached to thevalve 10, etc. Those of ordinary skill in the art understand the variousspecified operating conditions for a particular valve 10.

For example, a gas used in a typical installation might be air, naturalgas, carbon dioxide, propane, or another gas. Temperature duringoperation of the valve 10 may be 100 degrees F. at the inlet nozzle 25of the valve 10 and 70 degrees F. at the secondary orifice 30. A typicalset pressure might be 100 psig, and the typical outlet piping 50 mightbe 2-inch schedule 80 piping that is 20 feet long and has one elbow.

For a particular installation, certain operating conditions may involveranges. For example, a valve may be specified to operate at anytemperature within a range of temperatures. Often, the manufacturer of avalve will specify ranges of operating conditions for which their valvesare designed. For purposes of determining the specified operatingconditions for the purposes of interpreting the attached claims, testsshould be run at the midpoint of any such ranges.

Because the specified operating conditions vary for differentinstallations, the desired area of secondary orifice 30 for a particularinstallation must be determined by one of skill in the art throughroutine experimentation given the specified operating conditions. Anexample is provided hereafter to assist one of skill in the art indetermining the area of the secondary orifice 30 in the valve 10.

When the ideal area for secondary orifice 30 is determined andimplemented in a valve 10, gas flows from the chamber 20 through thesecondary orifice 30 in a sonic flow when the valve is open and runningunder specified operating conditions and there is at least 30 psia ofpressure in the chamber 20. The sonic flow through the secondary orifice30 prevents built-up downstream back pressures caused by the length andshape of the outlet piping 50 from affecting the blow-down value of thevalve 10. Thus, the snap-type safety relief valve 10 may maintain itsblow-down value better, chatter less, and last longer.

It is preferred that outlet piping 50 be less than 20 feet. It is alsopreferred that outlet piping 50 have 0, 1, or 2 elbows. Through routineexperimentation, one of skill in the art can determine at what lengthand with how many elbows that the outlet piping 50 begins to affect theblow-down value of the snap-type safety relief valve 10 by preventingthe flow out of the secondary orifice from being sonic.

The American Society of Mechanical Engineers Boiler and Pressure VesselCode recommends that built-up downstream back pressures (such as thosecaused by outlet piping 50) for standard spring-operated valves does notexceed 10% of the set pressure at the outlet chamber 47.

Referring to FIG. 2, another preferred embodiment of a snap-type safetyrelief valve 110 is shown. Valve 110 is substantially identical to valve10. The only difference is that valve 110 does not have an orifice plate40. Instead, the body 118 of valve 110 is shaped to provide the functionof creating the area of secondary orifice 130 through which gas flows ina sonic manner. The body 118 can be so shaped during its manufacture.The particular dimensions of the secondary orifice 130 depend upon theparticular specified operating conditions of the installation, and mustbe determined by one of skill in the art through routineexperimentation.

Importantly, the secondary orifice 130 should be sized so that thepressure in the outlet chamber 147 is less than about 50% of thepressure inside the chamber 120 when the valve 110 is opened due to apressure exceeding the set pressure, and the valve 110 is running underspecified operating conditions, and there is at least 30 psia ofpressure in the chamber 120. This ensures sonic flow through thesecondary orifice 130, preventing built-up downstream back pressuresfrom affecting the blow-down value of the valve 110.

Referring to FIG. 3, another preferred embodiment of a snap-type safetyrelief valve 210 is shown. Valve 210 is substantially identical to valve10, except that the valve 210 has a chamber 220 into which a hollowcylinder 245 has been inserted. The volume inside the hollow cylinder245 serves as part of the wall of the chamber 220 in valve 210. That is,to achieve sonic flow from the inlet chamber 216 into the hollowcylinder 245, the pressure inside the hollow cylinder 245 must be lessthan about 50% of the pressure in the inlet chamber 216.

Also in FIG. 3, a sidewall of the hollow cylinder 245 has a secondaryorifice 230 defined therein. Gas must escape through secondary orifice230 before entering the outlet piping 250. The particular dimensions ofthe secondary orifice 230 depend on the specified operating conditionsfor a particular installation of the snap-type safety relief valve 210,and must be determined by one of skill in the art through routineexperimentation.

Importantly, the secondary orifice 230 should be sized so that thepressure in the outlet chamber 247 is less than about 50% of thepressure inside the chamber 220 when the valve 210 is opened due to apressure exceeding the set pressure, and the valve 210 is running underspecified operating conditions, and there is at least 30 psia ofpressure in the chamber 220. This ensures sonic flow through thesecondary orifice 230, preventing built-up downstream back pressuresfrom affecting the blow-down value of the valve 210.

Referring to FIGS. 4 and 5, a preferred inlet nozzle 301 is shown. FIG.4 shows a cross-sectional view of the inlet nozzle 301, and FIG. 5 is atop view of the same. The inlet nozzle 301 can be used with any lowblow-down snap-type relief valve. The inlet nozzle has as huddlingchamber 303. Huddling chambers are described in U.S. Pat. Nos.4,446,886; 4,566,486; 4,799,506; and 4,932,434; which are hereinincorporated by reference in their entirety.

Basically, a huddling chamber is a region in an inlet nozzle that causesa preferred snap-type safety relief valve to snap open. If a huddlingchamber is too large, it can cause a blow-down value of a snap-typesafety relief valve to be higher than desired. Thus, it may be desirableto modify a huddling chamber so that the valve still snaps open, but thevalve keeps a low blow down value.

Such modifications can be done by several methods that would beunderstood by one of skill in the art. For example, in FIG. 5, slots 305were milled into the nozzle 301 to temper the effect of the huddlingchamber 303. This way, some gas escapes through slots 305 rather thanacting against the disk member.

PROPHETIC EXAMPLE 1 Determining the Size of a Secondary Orifice

Referring to FIG. 6, a non-limiting example is provided to show apreferred method for experimentally determining what size to make asecondary orifice 430 in a trial snap-type low blow-down safety reliefvalve 410.

The trial snap-type low blow-down safety relief valve 410 is designedfor specified operating conditions. For example, the trial snap-type lowblow-down safety relief valve 410 may be designed to operate with amaximum length of from about 15 feet to about 20 feet of outlet piping450 having one elbow (not shown). The outlet piping 450 is of the typewith schedule number 80, since this is the most common seamless pipe forthe pressure ranges addressed in this non-limiting example. The trialsnap-type low blow-down safety relief valve 410 may also be designed fora set pressure of 100 psia that will relieve air from a pressurizedsystem when the pressures therein exceed the set pressure. In a typicalscenario, the air would flow into the valve 410 at 100 degrees F. andflow out of the valve 410 at 70 degrees F.

A trial snap-type low blow-down safety relief valve 410 having outletpiping 450 is mounted on a pressure vessel 415. The pressure in thepressure vessel 415 is reported by a vessel pressure gauge 480. Thepressure in the trial snap-type low blow-down safety relief valve 410 isreported by a body pressure gauge 470. Both gauges are set to readpressure in psia.

The pressure in the pressure vessel 415 is increased until the trialsnap-type low blow-down safety relief valve 410 snaps open. After thesnap-type low blow-down safety relief valve 410 is fully open, thepressure reading on the vessel pressure gauge 480 should be compared tothe pressure reading on the body pressure gauge 470.

As long as the pressure reading on the body pressure gauge 470 is lessthan about 50% of the pressure reading on the vessel pressure gauge 480,gas will flow sonically from the inlet nozzle 425 into the body of thetrial snap-type low blow-down safety relief valve 410. Preferably, thepressure reading on the body pressure gauge 470 is from about 35% toabout 50% of the pressure reading on the vessel pressure gauge 480. Itis preferred that while the valve 410 is fully open, the pressure in thebody should not drop below 30 psia.

This permits sonic flow of gas from the pressure vessel into the body ofthe trial snap-type low blow-down safety relief valve 410. This alsopermits sonic flow from the body of the trial snap-type low blow-downsafety relief valve 410 out of the secondary orifice 430 so long as theback pressure due to the length and number of elbows in the outletpiping 450 are less than about 50% of the pressure reading on the bodypressure gauge 470 and there is at least about 30 psia of pressure inthe body of valve 410. Then, gas escapes through the outlet piping 450and out to the atmosphere.

If the difference in the pressure readings falls within the 35 to 50%range, no adjustment need be made to the size of the secondary orifice430 on the trial snap-type low blow-down safety relief valve 410.

If the reading of the body pressure gauge 470 exceeds 50% of the readingof the vessel pressure gauge 480, the size of the secondary orifice 430should be enlarged, and the trial process repeated until the differencein the pressure readings falls within the preferred range. Otherwise,subsonic flow will occur at the inlet nozzle 425.

If the reading of the body pressure gauge 470 is less than 35% of thereading of the vessel pressure gauge 480, the size of the secondaryorifice 430 should be reduced, and the process repeated until thepressure readings falls within the preferred range. Otherwise, thelength of outlet piping 450 that the relief valve 410 can withstandwithout a change in blow-down value will be reduced.

When the critical pressure ratio of the gas being used in the system isread on the body pressure gauge, then the valve designer will know thatthe secondary orifice 430 is the smallest size possible withoutrestricting the flow at the inlet nozzle 425.

Also, the outlet piping 450 can be changed with other, longer outletpiping and/or outlet piping having more elbows, so long as theadditional length and/or elbows do not cause the pressure just outsidethe secondary orifice 430 to become greater than about 50% of thepressure reading on the body pressure gauge 470. However, if too muchoutlet piping 450 is added, the flow out of the secondary orifice 430will not be sonic and the blow down value of the valve 410 will beaffected.

In conjunction with the foregoing design for a low blow-down valve, amethod of manufacturing such valves has also been invented. As notedabove, in order to manufacture a series of valves having the designedrange of set rates, a solution to the problem of spring rate variabilitywas needed. First, a different type of spring arrangement was conceivedof. Second, to use low cost coil springs, and rather than discardingcoil springs outside of a small tolerance from the desired spring rate,ways of modifying the spring rate were developed.

One method for constructing a set point spring to a close rate toleranceis to use a different type of spring arrangement, such as a conicalshaped washer with a hollow center, as shown in FIG. 7. In thisembodiment, a stack of disc springs, particularly Belleville springwashers 538, is used to build a low rate tolerance set point spring.Belleville washers are available in a wide variety of diameters andmaterial thicknesses. These washers may be stacked in series as shownwhereby the overall stack spring rate is diminished as the number ofdisks is increased. Reducing the thickness of each disk also reduces thespring rate.

By selection of the diameters and thickness and the number in the stack,a very close spring tolerance can be obtained. Some disadvantages ofthis approach are hysteresis in the spring stack due to friction, costs,and the large inventory of thicknesses required to reach a desiredspring rate. Thus, while workable, other approaches, such as modifyingthe spring rate, are more desirable.

One method of modification of a spring rate, sometimes used in pneumaticinstruments, is by making a plug member with an internal spiral groove.The spiral groove is designed with a pitch roughly equal to the pitch ofa wound coil spring with one end not “closed and ground.” The coilspring is then designed to have a rate less than that desired. Since therate of a coiled spring follows the following formula:${{Rate}\quad R} = \frac{G\quad d}{8\quad n\quad D}$ WhereG = Modules  in  Torsion d = Wire  outside  diametern = No.  of  active  coils D = Mean  diameter  of  coils.The spiral plug is then adjusted into the spring to short out a numberof active coils, which increases the spring rate. By varying theinsertion depth of the plug, the rate of the spring can be adjusted froma low design value to the desired value to give the required 5% to 10%blow-down. One disadvantage of the system is the extremely closetolerance of the internal groove. This arrangement is shown in FIG. 8,which illustrates the use of a standard machine wound coil spring withon end “as wound” i.e. not closed and ground square. A hollowcylindrical member 639 with an internal spiral groove is threaded overthe lower end, effectively shorting out one or more coils of the coilspring. The lower face of the cylindrical member rests on washer 641,which in turn rests on the upper face of disk 628. Threading thecylindrical member 639 onto the spring in varying degrees effectivelyadjusts the number of active coils in the coil spring, which theneffectively adjusts the rate of the spring. By this method, thetolerance on spring rates can be reduced from a standard ±7% to atighter value of ±2%.

Four new designs for spring rate modification have been invented whichare particularly preferred. These methods are shown in FIGS. 9 through11. These four designs are based on the use of a standard wound coilspring having a rate tolerance of +7% which is then modified to reducethe rate tolerance to +2%. The +2% rate tolerance is required to achievethe previously stated advantages.

FIG. 9 illustrates another preferred embodiment of the invention whichutilizes a standard wire wound spring 738. A washer 743 is stacked onthe end of spring 743 and engages a short stack of Belleville washers742, which are arraigned as shown. These Belleville washers effectivelyreduce the rate tolerance of +7%, which would be the case if noBelleville washers were used, to a rate tolerance of +2%. The followingdiscussion will illustrate design of this method.

This design involves using standard coil spring and adjusting them withdisk springs. This allows a spring that has a high rate to be adjusteddown to a desired lower rate. If the rate is within a small percentageof its total rate, such as +7%, then only a few disk springs are neededto do this.

To understand how these embodiments work, a better understanding of howa combination of springs work, and how disk springs themselves work, isneeded. Using springs in combination is a way to change the spring ratein a very predictable fashion. Springs can be used in many differentcombinations but are simplified into two basic groups, series andparallel. Series combinations decrease the spring rate by placing thesprings on top of each other. This can be represented mathematically bythe following equation${\frac{1}{R_{eq}} = {\frac{1}{R_{1}} + \frac{1}{R_{2}} +}},{\ldots\quad\frac{1}{R_{n}}}$where R_(eq) is the new combined rate; R₁ R₂ and R_(n) are the springsin series. Parallel combinations are when springs are placed side byside, which increases the total combined spring rates. This also can berepresented by the equation R_(eq)=R₁+R₂+, . . . R_(n) where R_(eq) isthe new combined rate; R₁, R₂ and R_(n) are the springs in parallel.

After looking at the two equations is can be seen that any spring ratecan be changed to a desired rate. If a spring is not at its desired ratethen a second spring could be used in combination to achieve the desiredrate with relative accuracy depending on the accuracy of the measuredrate of the initial two springs. In application this technique may notbe practical due to space constraints of the individual application. Forexample to cut a spring's rate in half a spring of the same rate isplaced on top of the original spring, or in series, but the total heightof the spring combination also increases by two. The same space problemfor parallel combinations is also true, with the new spring taking upspace next to the original spring.

Disk springs, such as Belleville washers, could be used instead of coilsprings. This type of disk spring is like a washer that's had its middlepushed up. This type of spring is not as common as a coil spring, but isstill used quite frequently. One of its most common applications is as awasher that applies force when used under the head of a bolt. As thebolt is tightened the disk spring flattens, flexing the spring, thusapplying force to the head of the bolt, and stops the bolt from backingout. Disk springs can also be used like coil springs when they arestacked vertically in a column and then compressed together all as onesingle spring. This is quite handy because the rate of the stackedspring is dependent on the number of disks and how they are arranged.Disk springs used in columns however do have their disadvantages, theyare normally used only when small displacements are needed, and accurateguidance over the entire length of the column is essential. Disk springsrates are dependent on the geometry of the disk and are linear forheight-to-thickness ratios of around 0.4. Disk springs have a tolerancethat is slightly higher than that of coil springs, around 20%. Disksprings do not handle dirty environment as well as coil springs becauseeach disk rubs against the next disk as they flex. This can make thespring rate change due to an increase in fiction or reduce the life ofthe spring because of wear. The overall accuracy and consistency of therate of a column of disk springs is much less than that of coil springsdue to this friction. Therefore stacked disk springs are not used aspreferred as other embodiments of the present invention.

A technique has been found to combine two of the previous methods thatallows one to adjust rate and take up as little space as possible, andstill not affect the integrity of the coil spring. This is ideal for anyapplication that has a spring that is working in line with the appliedfore. This technique involves using standard coil springs and adjustingthem with disk springs. This allows a spring that has a high rate to beadjusted down to a desired lower rate. If the rate is within a smallpercentage of its total rate, such as ±7%, then only a few disk springsare needed to do this.

As stated before, the spring rate of a column of disk springs isdependent on the number of disks and their arrangement. These disksprings can be arranged so that they work in parallel or series. Thisallows combinations of one size disk spring to be used to correct onesize coil spring. This reduces cost because an inexpensive coil springcan be corrected by a few very inexpensive disk springs.

A single disk spring like a coil spring has a rate, which is dependenton deflection and load. These disk springs, unlike coil springs, can bearranged in a quite unique manner that allows both parallel and seriescombinations to be achieved in a vertical column. To explain this it isbest to look at the simple case, which involves only two disks springs.If both disks are stacked with the same orientation we find that therate has increased by two. This is an example of parallel stacking. Ifthe disks are stacked facing in opposite directions we find that therate has been reduced by half, this would be an example of a seriesstack. This series and parallel stacking works for any number of disks.If we look at more than just two springs we quickly find that there aremany more different combination possible. This allows for one size diskspring to be used in multiple column arraignments, which have manydifferent rates.

The combination of disk springs and coil spring can be broken up into totwo separate springs that are in series. First the coil spring or themain spring, and then the column of disk springs that are called theadjustment springs. Because the main and adjusting springs are inseries, the new combined rate is always lower than that of the rate ofjust the main spring. This means that the main spring can only beadjusted down, not up with this technique. This will affect the designof the rate and tolerance of the main springs. If the spring rate canonly be reduced then the main springs should be designed with a minimumtolerance of the rate needed, and a rate tolerance drift that is onlypositive. For example if a rate of 100 (lb/in) were needed, then thespring would be designed to have a minimum rate of 100 (lb/in) and apositive tolerance of 10%, or a rate that is between 100-110 (lb/in).The main spring can then be lowered by the adjustment springs until itis within a new tighter design tolerance.

The disk spring must be designed so the coil spring can be adjusted witha minimum amount of height change; this means that the use of as fewdisk springs is essential. To make this easier the disk springs are madeto change the coil springs rate by a desired percentage. The percentagechange chose for a single disk spring is 0.2% to make things as simpleas possible. This means that an ideal coil spring and an ideal singleadjustment spring together have a combined rate of 98% of the coilspring by itself. The disk springs can then be arranged so that the ratecan be adjusted from 1% to 10%. The following is a table of differentseries and parallel combinations and their combined rates. TABLE 1 DiskSpring Arrangements % Rate Number New Disk Change of Coil of Old Coil %of combined Arrangements Spring Disks Ideal Rate Rate 5S 10%  5 110%100% 2P-4S 9% 6 109% 100% 4S 8% 4 108% 100% 2P-3S 7% 5 107% 100% 35S 6%3 106% 100% 2P-2S 5% 4 105% 100% 2S 4% 2 104% 100% 2P-1S 3% 3 103% 100%1 2% 1 102% 100% 2P 1% 2 101% 100% 0 0% 0 100% 100%S—SeriesP—Parallel

From this table any coil spring with a rate between 100%-110% of thedesign rate can be adjusted down to the goal rate.

Other design factors are the size and dimensions of the disk spring.This is dependant on space requirements and the working range of thedisk spring. The space requirements depend on the applications thatinclude ID and OD dimensions. The disk spring is also designed so thatthe disk is deflected 50% of total solid displacement at the workingload. This allows room for the disk to move and ensure that it does notgo to its solid height. Thickness of the disk should be chosen to allowfor the thickness to height ratio to be as close to 0.4 as possible. The0.4 ratio will make the disk spring as linear as possible. In someapplications this may not be possible, but because the disk springs areonly used as slight adjusters, this nonlinear rate has only a smalleffect on the linear rate of the main coil spring. Guidance is anotherfactor that must be considered when using this technique. The disksprings must be contained so when they are flexed their rate will stayas consistent as possible. The most practical method of guidance is witha center alignment guide. With this type of guidance the disks and mainspring are designed to have the correct clearance on the center guidanceshaft. Both disk and coil springs are free to be compressed with asminimal friction as possible. Guidance may also be achieved with anexterior guide, which is less desirable because the disk spring OD tendsto be larger than the coil spring. Exterior and interior guidance canalso be used together with the disk being guided on the outside and thecoil spring guided on the inside.

The following is a step-by-step example of how this technique isapplied. For this example a target rate has been chosen of 100 (lb/in)and an allowable tolerance of ±2%.

1. First the main spring is designed having a rate of 108 (lb/in) with atolerance of ±7%. This is a standard inexpensive coil spring that has astandard rate tolerance. The reason why the rate of 108 (lb/in) ischosen is to ensure that the main spring will not drift below 100(lb/in) target rate.

2. An adjustment disk spring is then chosen by finding a rate that willadjust the main spring by 2%. This is done to allow for the minimumamount of adjuster spring combinations. The nominal spring rate of themain spring is 108 (lb/in) and if we adjust this by 2% we then have arate of 105.84 (lb/in). We need a disk spring that adjusts this rate by2% so we can use the equation for series springs to find this.$\frac{1}{105.84\left( {{lb}\text{/}{in}} \right)} = {\frac{1}{108\left( {{lb}\text{/}{in}} \right)} + \frac{1}{R_{disk}}}$

This is then solved for R_(disk)=5,292 (lb/in). A disk spring with arate of 5,292 (lb/in)±15% can then be ordered that has an OD and IDdimension that best fits the space allowances of the application.Looking at the two spring rates we notice that 108 is approximately 2%of 5,400, which lends itself to a very quick and easy approximateformula. $R_{disk} \cong \frac{108}{2\%}$

3. When the disk springs are designed one other factor must also betaken into consideration. The load at which the main coil springachieves maximum stress must be equal to or less than the load at whichthe disk springs reach their maximum stress. This will allow the coilspring to be used at its intended deflection ranges without overstressing the disk springs.

4. Next we measure the coil spring to find the exact spring rate. Fromthis and the series formula we can find what rate the adjusting springrate needs to be. If the coil springs rate is measured and found to be110 (lb/in) we can then find the adjustment rate.$\frac{1}{100\left( {{lb}\text{/}{in}} \right)} = {\frac{1}{110\left( {{lb}\text{/}{in}} \right)} + \frac{1}{R_{adjment}}}$

From this equation we find that R_(adjust)=1100(lb/in).

5. We now arrange the disk springs in a way that will produce theclosest match for the desired adjustment rate. The best arrangement is 5disk springs in series. This makes the new adjustment rate$\frac{1}{R_{adjust}} = \frac{5}{5292\left( {{lb}\text{/}{in}} \right)}$so R_(dajust)=1058.4 (lb/in).

6. We then can calculate the new combined rate for the spring assemblywith the series equation.$\frac{1}{R_{combined}} = {\frac{1}{110\left( {{lb}\text{/}{in}} \right)} + \frac{1}{1058.4\left( {{lbs}\text{/}{in}} \right)}}$

Which gives R_(combined)=99.65 (lb/in)

7. The new calculated combined spring rate is 0.36% low but it is stillwithin the desired ±2% of the 100 (lb/in) target rate. The individualdisk springs are also going to have a tolerance, which will make themeasured spring value slightly different than that of the calculatedvalue. The rate tolerance of each individual spring disk will only beapplied to the small amount of adjustment. This means that if the mainspring was adjusted down by 10% and the adjustment disk springs had arate tolerance of ±15%, then the final combined rate would have a 10% ofa ±15% tolerance or ±1.5% total tolerance. The new combined rate of theabove example spring assembly is now 99.65 (lb/in)±1.5%.

Each spring that is adjusted is brought into tolerance by the disksprings only by the amount of the original error in the main spring.This will make the total error in the combined springs dependent on theamount of initial coil spring error. If a main-spring was dropped by 2%to achieve a final rate then the error of the adjustment springs willonly affect the combined rate by 2% of the disk spring tolerance. As themain springs become further out of tolerance then the disk spring errorwill effect the final combined rate tolerance that much more.

In the above application of the low blow-down safety relief valve thishas proven to work extremely well. The spring rate can be chosen for aparticular valve and then is ordered with standard tolerances. Thisspring then can be measured and then adjusted with disk springs veryquickly. Each different spring ordered can be adjusted by using a chartlike Table 1 to quickly adjust the spring down to the desired rate. Inthe valve application the coil spring was originally guided by anadjustment screw running the spring's center. The disks springs are madewith the center hole bigger to allow for clearance and a bushing aboveguides the disk springs on there outside diameter. The disks actdirectly on top of the spring, which allows for minimal vertical spacerequirements. The first disk spring that is above the main spring mustbe placed in a manner that allow the center of the disk spring to be incontact with the end of the main spring and the outside diameter must behigher than the end of the main spring. This will dictate how the disksprings will be arranged above the main spring.

FIGS. 10 and 11 illustrate additional preferred embodiments of thisinvention. Reducing the wire cross sectional area produces a reductionin spring rate. This is done by machining or grinding the outside orinside of a standard spring until the desired rate is achieved. Thischanges the springs normal round cross section to one that has a flat onone side. This may make the spring more susceptible to corrosion due tothe exposed surface of the spring. Also, coil springs are heat treatedwhen they are made so the heat generated in this operation may affectthe internal stress of the spring wire. As shown in FIG. 10, a standardwound coil spring having a rate tolerance of ±7% can readily be reducedto ±2% by removal of metal from the outside diameter of the spring. Forexample, if a set spring was required to have a spring rate of 100lbs/in. ±2%, a coil spring would be procured having a design rate of 107lbs/in. Then, the spring lot would be carefully measured to determinethe statistically accurate mean rate. Note that the actual variation ofspring rates within a batch of springs wound at the same time is small,perhaps a ±2% variation. However, the next batch of springs wound with adifferent batch of spring wire and a slightly different spring windingmachine set up would produce as much as a ±7% variation from the nominalof 107 lbs/in. rate. The statistical mean is then used as a reference todrop the rate of the batch of springs. If each of the springs of theinitial batch is within the small ±2% then the modified springs shouldstill be within ±2% of each other but at a new lower mean rate.

Equations for predicting how much of a reduction in rate could beproduced with the use of test springs that a reduced incrementally andmeasured between each reduction. This data then can be graphed and anequation can then be generated that can predict a reduction in rate forany reduction in spring diameter. Equations for stress are available,which must be kept within the elastic limit of the spring material.Metal removal from the inside or outside diameter of a coiled spring canbe accomplished by a variety of methods such as grinding, boring,machining, etc.

FIG. 12 shows an additional preferred embodiment of this invention. Thisdesign utilizes a standard coil spring in which the spring ratetolerance is reduced by manually enlarging the mean spring diameter ofseveral of the central coils. Uncoiling a coil spring will increase themean diameter and reduce the number of coils. This reduces the springrate because of the increase in mean spring diameter. Uncoiling alsoincreases the ID and OD dimensions of the spring, which makes thistechnique less desirable. The spring also has added stress do to theuncoiling operation, which may require a stress relieving operation. Ifa spring rate of 100 lbs/in. ±2% were required, a lot of springs wouldbe wound to a 107 lbs/in. ±7% rate tolerance. The central coils wouldthen be expanded mechanically by an expanding internal mandrel, or atorquing action applied to the ends of the spring, tending to unwind theoriginal coil. Enlarging the mean spring diameter or a group of centralcoils would bring the actual spring rate to the desired 100 lbs/in. ±2%.

It should be appreciated that the apparatus and method of the presentinvention is capable of being incorporated in the form of a variety ofembodiments, only a few of which have been illustrated and describedabove. For example, the modified springs can be used in other safetyrelief valves besides those disclosed herein. The invention may beembodied in other forms without departing from its spirit or essentialcharacteristics. The described embodiments are to be considered in allrespects only as illustrative and not restrictive, and the scope of theinvention is therefore indicated by the appended claims rather than bythe foregoing description. All changes that come within the meaning andrange of equivalency of the claims are embraced to be within theirscope.

1. A safety relief valve comprising: a) a body comprising a chambertherein and an inlet and an outlet, the inlet comprising an inlet valveseat; b) a disk member closable on the inlet valve seat; and c) amechanism in the body biasing the disk member to rest on the inlet valveseat with a set force such that when the pressure in the inlet exceeds aset pressure resulting from the set force, the disk member is liftedfrom the inlet valve seat, the mechanism comprising a combination of acoil spring and one or more disk springs stacked in series with the coilspring.
 2. The safety relief valve of claim 1 further comprising asecondary orifice between the chamber and the outlet, the secondaryorifice being sized so that gas flows from the inlet valve seat into thechamber in a sonic flow and so that gas flows from the chamber throughthe secondary orifice in a sonic flow when the valve opens due to apressure in the inlet exceeding the set pressure.
 3. The safety reliefvalve of claim 1 wherein the mechanism further comprises an adjustmentscrew acting against the combination of coil spring and one or more disksprings.
 4. The safety relief valve of claim 3 wherein the adjustmentscrew has a bore therein that receives a stem on the disk member whenthe disk member is lifted from the inlet valve seat.
 5. The safetyrelief valve of claim 1 having a blow-down value of less than about 10%.6. The safety relief valve of claim 1 having a blow-down value of lessthan about 5%.
 7. The safety relief valve of claim 1 comprising aplurality of disk springs.
 8. The safety relief valve of claim 7 whereinat least two of the disk springs are stacked to operate in series. 9.The safety relief valve of claim 7 wherein at least two of the disksprings are stacked to operate in parallel.
 10. The safety relief valveof claim 7 wherein there are at least three disk springs, and at leasttwo of those are stacked facing in an opposite direction to the thirddisk spring.
 11. The safety relief valve of claim 2 wherein thesecondary orifice is formed in a plate that is inserted into the body ofthe safety relief valve.
 12. The safety relief valve of claim 2 whereinthe secondary orifice is formed integral with the body of the safetyrelief valve.
 13. A method of manufacturing a group of safety reliefvalves each with a blow-down value of less than about 10%, the groupcovering a range of set pressures, the method comprising: a)manufacturing multiple bodies, each having a chamber within the body, aninlet and an outlet, the inlet comprising an inlet valve seat; b)fitting a disk member within each of the bodies so as to be closeable onthe inlet valve seat within the body; c) obtaining a plurality of coilsprings to be used in the group of safety relief valves, each springhaving an original spring rate, the original spring rates varying withinthe plurality of springs; d) measuring the original rate of each spring;e) assembling the valve by placing a given spring within a given body soas to act with an adjustable force on the disk member, the given springhaving its original spring rate modified prior to the final assembly ofthe valve.
 14. The method of claim 13 wherein the spring rate ismodified by having one or more disk springs stacked in series with thecoil spring.
 15. The method of claim 13 wherein the spring rate ismodified by shorting out a portion of the coils of the spring.
 16. Themethod of claim 13 wherein the spring rate is modified by having aportion of the surface of the spring removed.
 17. The method of claim 16wherein the spring has material removed from its inside diameter. 18.The method of claim 16 wherein the spring has material removed from itsoutside diameter.
 19. The method of claim 13 wherein the spring rate ismodified by mechanically enlarging the internal diameter of the spring.20. A snap-type safety relief valve designed for use under specifiedoperating conditions on a pressure vessel or a flow line having apressurized gas therein, the safety relief valve comprising: a) a bodyattachable to the pressure vessel or flow line, the body comprising achamber therein and an inlet and an outlet, the inlet comprising aninlet valve seat, the outlet being adapted to attach to outlet piping;b) a disk member closable on the inlet valve seat; c) a mechanism in thebody comprising a spring having a modified spring rate biasing the diskmember to rest on the inlet valve seat with a set force such that whenthe pressure in the pressure vessel exceeds a set pressure resultingfrom the set force, the disk member is lifted from the inlet valve seat;and d) a secondary orifice between the chamber and the outlet, thesecondary orifice being sized so that gas flows from the inlet valveseat into the chamber in a sonic flow and so that gas flows from thechamber through the secondary orifice in a sonic flow when the valveopens due to a pressure in the pressure vessel or flow line exceedingthe set pressure during testing under the specified operatingconditions.
 21. The relief valve of claim 20 wherein the spring rate hasbeen modified by having one or more disk springs stacked in series withthe coil spring.
 22. The relief valve of claim 20 wherein the springrate has been modified by shorting out a portion of the coils of thespring.
 23. The relief valve of claim 20 wherein the spring rate hasbeen modified by having a portion of the surface of the spring removed.24. The relief valve of claim 20 wherein the spring rate has beenmodified by mechanically enlarging the internal diameter of the spring.25. The relief valve of claim 21 wherein the one or more disc springshave a hole in their center, and the mechanism comprises an adjustmentscrew extending through the holes in the one or more disk springs.
 26. Acombination of outlet piping and a snap-type safety relief valvedesigned for use under specified operating conditions on a pressurevessel or flow line having a pressurized gas therein, the combinationcomprising: (a) a safety relief valve body attachable to the pressurevessel or flow line, the body comprising a valve body chamber thereinand an inlet and an outlet, the inlet comprising an inlet chamberupstream of an inlet valve seat, the outlet being attached to the outletpiping, a portion of the outlet piping creating an outlet chamberproximal to the outlet; (b) a disk member closable on the inlet valveseat; (c) an adjustment screw acting on a spring having a modifiedspring rate in the body biasing the disk member to rest on the inletvalve seat with a set force such that when the pressure in the pressurevessel exceeds the set pressure resulting from the set force, the diskmember is lifted from the inlet valve seat; and (d) a secondary orificebetween the valve body chamber and the outlet, the secondary orificebeing sized so that pressure in the valve body chamber is less thanabout 50% of the pressure in the inlet chamber and so that pressure inthe outlet chamber is less than about 50% of the pressure in the valvebody chamber, when the valve opens due to a pressure in the pressurevessel or flow line exceeding the set pressure during testing under thespecified operating conditions.
 27. An improved low blow-down snap-typesafety relief valve having an inlet, a disk member closable on theinlet, a mechanism biasing the disk member on the inlet, a body, and anoutlet, the improvement comprising the valve preventing built-updownstream back pressures from changing its blow down value by includinga secondary orifice in the body of the safety relief valve sized topermit gas to escape from the body through the secondary orifice in asonic flow when the valve is opened during testing under operatingconditions specified for the use of the valve, and the mechanismcomprises a spring having a modified spring rate. 28-34. (canceled) 35.An improved low blow-down safety relief valve having an inlet, a diskmember closable on the inlet, a mechanism biasing the disk member on theinlet, a body, and an outlet, the improvement comprising the mechanismcomprising a spring having a modified spring rate.